1. Field of the Invention
The present invention relates to a process for generating a magnetic induction field within a magnetic medium or environment, a magnetic field generating device for application of this process and a magnetic recording system incorporating this device. The invention is more particularly applicable to the recording of data on a magnetic carrier and consists in magnetizing the carrier which is positioned in front of and close to a magnetic field generator device, more commonly referred to as a recording transducer, by means of the magnetic field generating device.
2. Description of the Prior Art
In order to better to understand the object of the invention, a brief review of principles in respect to magnetism may be helpful. To this end, it is known that in order to magnetize a magnetic material or a magnetic medium, the material is first exposed to a positive magnetic field having a strength H.sub.s sufficient for the material to be saturated, that is to say for the magnetic induction in the material to reach a limiting value B.sub.s. The magnetic field is then removed. This then leaves in the material a magnetic induction (+B.sub.r) differing from zero value which is referred to as the remanent induction, and which is characteristic of the particular magnetic material. The magnetic material is then said to be saturated. In other words, magnetizing a magnetic material amounts to saturating the same magnetically. It will be recalled furthermore that if a negative magnetic field is applied to this material, the magnetic induction in the material is cancelled for a value of H referred to as the coercitive field H.sub.c. The graph illustrating the variation of the magnetic induction as a function of the field H is then referred to as the "major hysteresis cycle" of the magnetic material.
If the magnetic material is exposed to a magnetic field whose strength is lower than H.sub.s and if this field is then removed, a magnetic induction having a value smaller than B.sub.r then remains within the material. The magnetic material is then said to be unsaturated. If a negative magnetic field is then applied to the same, the magnetic induction is cancelled for a value of H which will be referred to as H.sub.cm. The graph illustrating the variation of the induction is then referred to as the "minor hysteresis cycle". It should be evident that, for a given magnetic material, there can be an infinite of minor hysteresis cycles between the major cycle on the one hand, and the limiting minor cycle for which the induction and the magnetic field constantly remain equal to zero. The form of the minor cycles varies from one cycle to another; i.e., the minor cycles are not deduced from each other by similarity.
The totality of the magnetic properties of a given point of a magnetic material, that is the induction value, cycle form, coercivity field value, etc., defines the magnetic state of the material at this point. This state is also referred to as magnetization at this point. As a rule the magnetic state of each of the points of a material or medium is not necessarily identical at a given instant.
A magnetic material or medium in which the induction differs from zero (saturated or not) creates a magnetic leakage or stray field in the immediate vicinity of each of the points of its surface, with respect to which field it is demonstrable that it is a function of the coercivity field at this point, and is always smaller than the latter in practice. A medium whose magnetic state may be modified by applying stresses to the same (traction or compression), is referred to as a magnetostrictive environment. The corresponding physical action is referred to as magnetostriction.
Research regarding the phenomenon of magnetostriction and its applications are described, for example, in the following works or articles, an understanding of which would be helpful to an understanding of the invention.
(a) "Research On A Ferro-acoustic Information Storage System", J. W. GRATIAN, R. W. FREYTAG, NASA REPORT CR 249 PA1 (b) "Compositional And Angular Dependence Of The Magnetostriction Of Thin Nichel Films", E. N. MITCHELL, G. I. LYKKEN, G. T. BABCOCK, published in J.A.P. (Journal of Applied Physics) volume 34--No. 4--Part 1--April 1963. PA1 (c) "Wire-type Acoustic Delay Lines For Digital Storage", SCARROT, G. G. NAYLOR, published in PROC, IEEE, Part B, Suppl. 3, vol. 103, April 1956, pages 497 to 508. PA1 (d) SONISCAN--"A New Memory Device", by E. U. COHLER and H. RUBINSTEIN published in IEEE "Transactions on Magnetics" Vol. MAG. 2, No. 3, September 1966, pages 528/529.
Magnetic carriers used for data recording are most frequently either cylindrical drums or rigid or flexible discs, or else tapes. They are generally of two types:
the longitudinal magnetization carriers in which the direction of magnetic induction is parallel to the surface of the carrier; and
the perpendicular magnetization carriers in which the direction of magnetic induction is perpendicular to the surface of the carrier.
A longitudinal magnetization carrier is commonly associated with a device for generating a magnetic field, formed by an electromagnet whose air gap is a narrow slot. The length of this slot may range between several microns and several tens of microns. When a current is caused to flow in the winding of the electromagnet, the magnetic field lines which close the magnetic circuit of the electromagnet between it poles, outside the air gap, constitute magnetic leakages close to the same, defining a magnetic leakage field. The magnetic carrier is exposed to this leakage field for the purpose of being magnetized. To record data on a carrier of this kind, the winding may be supplied with a current of variable intensity which creates on the carrier a series of small magnetic areas referred to as "elementary magnets" whose size is of the order of the length of the air gap.
In a more general manner, any surface or volume of a magnetic medium whose dimensions lie between a few microns and several hundreds of microns will be referred to as a magnetic area.
A carrier of the perpendicular magnetization type is commonly associated with a magnetic field generation device of the kind described in the French Pat. No. 2298850 and its corresponding U.S. Pat. No. 4,138,702.
A magnetic recording device of the type described in the aforenoted patent comprises an electromagnet and a magnetic shunt, the magnetic shunt and the electromagnet being positioned at either side of the perpendicular magnetization recording carrier and close to the same, so as to form a closed magnetic circuit in which the magnetic field lines are perpendicular to the surface of the carrier. The magnet comprises a recording pole and a flux closure pole, the crosssection of the recording pole being smaller than that of the flux closure pole. The dimensions of the elementary magnetic areas recorded on the carrier by the magnetic generation devices of this type is of the order of one to a few hundred microns.
It should be apparent that, depending on the kind of carrier and consequently on the kind of magnetic field generation device employed, the dimension of the elementary magnetic areas recorded will be of the order of a few microns to a few hundred microns. When the magnetic carrier is a drum or a tape, the data is recorded on a plurality of adjacent recording lines parallel to the generatrices in the case of a drum and to the width in the case of a tape. The dimension of these lines is of the order of one to several centimeters, or say several tens of centimeters. When the carrier is a disc, the data is recorded on circular concentric tracks having a radial width no greater than a few hundredths of a millimeter and covering the central portion the discs. The radial width of the totality of these tracks is of the order of a few centimeters. In order to record the whole of the data of a recording line of a drum or tape, or for recording data on the radial width of the whole of the tracks of a disc, two solutions are generally employed, notwithstanding the kind of magnetization adopted for these carriers.
The first solution consists in making use of a limited number of recording transducers, most frequently only one, which is displaced along the line which is to be recorded (in the case of a drum or tape) or radially throughout the width of the totality of the tracks (in the case of discs). This requires a controlled system for displacement and positioning of the transducer, which is accurate and costly.
The second solution consists in making use of a sufficient number of recording transducers in such manner that the data are recorded simultaneously throughout the recording line (or throughout the radial width of the totality of the tracks) without having to displace the transducers. This latter approach has several disadvantages. For example, the production cost of an assembly comprising a large number of transducers (several hundred) is high; and the associated electronic control circuits are complex and relatively expensive.